As home solar power generation systems proliferate, their cost is decreasing. FIG. 1 is a view showing the arrangement of a typical home solar power generation system.
Referring to FIG. 1, a DC power output from a solar battery 1 is converted into an AC power by a system interconnection inverter (to be simply referred to as an “inverter” hereinafter) 8 whose inputs and outputs are non-insulated, and connected to a single-phase three-wire 200-V system (to be simply referred to as a “system” hereinafter) 9 whose median potential line (to be simply referred to as a “neutral line” hereinafter) is grounded by a ground line 91 of a pole mounted transformer.
When an inverter having non-insulated inputs and outputs is used for a system interconnection solar power generation system, the solar battery 1 and system 9 are non-insulated. For this reason, the potential-to-ground of the solar battery 1 is fixed, and a ground fault current flows between one conductor and ground, like a ground fault on the AC side. In order to detect a ground fault at the solar battery 1, the inverter 8 has a current-detection-type ground fault sensor 89.
The power circuit of the inverter 8 is formed as a single-phase two-wire 200-V output to reduce the cost. For this reason, between the inverter 8 and the system 9, the neutral line is used only to detect the voltages of the remaining two lines, and no current flows to the neutral line.
Along with the recent expansion of the application range of solar power generation systems, connection to a single-phase 100-V system is required. To most easily meet this requirement, a non-insulated inverter with a single-phase two-wire 100-V output is connected to a single-phase 100-V system. However, development cost is necessary to newly develop a non-insulated inverter with a single-phase two-wire 100-V output. It is therefore preferable to use an inverter having an inverter circuit which outputs a single-phase two-wire 200-V, i.e., a most popular commercially available inverter at present.
Since an inverter with a single-phase two-wire 200-V output is designed not to flow a current to the neutral line, it is impossible to connect one side (two wires for the O-phase and U- or V-phase) of a single-phase three-wire 200-V output to two wires of a single-phase 100-V system.
To do this, an insulated transformer (to be simply referred to as a “transformer” hereinafter) 10 is used, as shown in FIG. 2. With this arrangement, the inverter 8 with a single-phase two-wire 200-V output and a single-phase 100-V system 4 can be connected. The U- and V-phase terminals are connected to input terminals A and C, respectively, of the transformer 10. The O-phase terminal is connected to input terminal B of the transformer 10. However, this arrangement has the following problems.    (1) The ground fault sensor 89 assumes that the potential-to-ground of the solar battery 1 is fixed and cannot detect a ground fault between one conductor and ground at the solar battery 1 in the arrangement shown in FIG. 2.    (2) The transformer 10 is generally large, heavy, and expensive.
When an inverter with a single-phase two-wire 100-V output is used, the potential-to-ground of a DC circuit is fixed. However, depending on the type of an inverter with a single-phase two-wire 100-V output, if reverse connection on the AC side, i.e., an abnormal connection between a ground-side electrical wire N and a non-ground side electrical wire H occurs, an excessive leakage current is generated through an earth capacitance 11, and an operation error of the ground fault sensor 89 or trip of an electrical leakage breaker takes place. Especially, for a solar battery integrated with a metal roof, the earth capacitance 11 is large, and a measure for preventing the reverse connection is indispensable.